One conventional infrared human body detector which has a wider sensitive angle than ordinary infrared human body detectors and has been evaluated as excellent is disclosed in U.S. Pat. No. 4,644,147. The disclosed infrared human body detector has a sensitive angle which is 90 degrees on both sides of a central sensitivity axis of the detector. The disclosed infrared human body detector is suitably mounted on a flat wall or ceiling.
There has not been available an infrared human body detector having a sensitive angle ranging from 180 to 270 degrees, suitable in an arrangement as shown in FIG. 4 of the accompanying drawings for monitoring an entire region P around a rectangular house H with infrared human body detectors S, nor has there been an infrared human body detector having a sensitive angle ranging from 270 to 360 degrees for monitoring a large room with a high ceiling, a garden, or the like. To monitor these areas, a plurality of infrared human body detectors have to be combined with each other, but such a combination is very uneconomical. Specifically, for monitoring a corner (270 degrees) of a house with infrared human body detectors, it has heretofore been necessary to install two infrared human body detectors, each having a sensitive angle of 180 degrees, on respective surfaces that jointly make up the corner. This system is highly disadvantageous because the price of the required infrared human body detectors and the expenses needed to install them are about twice those which would be necessary to employ one infrared human body detector.
Monitoring a large room with a high ceiling, a garden, or the like with a conventional infrared human body detector D2, as shown in FIG. 11(b) of the accompanying drawings, having a sensitive angle of 180 degrees, is also highly disadvantageous in that the initial and running costs are about twice those of an infrared human body detector according to the present invention for the reasons described above.